Target for vaporizing under an electron beam, a method of fabricating it, a thermal barrier and a coating obtained from a target, and a mechanical part including such a coating

ABSTRACT

The invention relates to a composite target in the form of a bar made of ceramic powders and designed to be evaporated under an electron beam, the target comprising zirconia and at least one zirconia stabilizer. In characteristic manner, said target is wherein said zirconia stabilizer is at a molar content lying in the range 2% to 30% and wherein said zirconia is formed by more than 90% of a monoclinic phase. The invention is applicable to fabricating a ceramic thermal barrier of low thermal conductivity and high thermomechanical strength formed by evaporation under an electron beam.

The invention relates to a composite target in the form of a bar made upof ceramic powders for the purpose of being evaporated under an electronbeam, the bar comprising zirconia and at least one zirconia stabilizer,and the invention also relating to a method of fabricating it.

The present invention also relates to making a thermal barrier havinglow thermal conductivity and high thermomechanical strength out ofceramic formed by evaporating such a target under an electron beam.

The invention also relates to a ceramic coating comprising such athermal barrier, and to a superalloy mechanical part including such acoating.

BACKGROUND OF THE INVENTION

The desire to increase the efficiency of turbomachines, in particular inthe field of aviation, and also to reduce fuel consumption and pollutingemissions of gases and unburned fuel, have led to fuel combustion underconditions that are closer to stoichiometric. This situation isaccompanied by an increase in the temperature of the gases leaving thecombustion chamber and heading towards the turbine.

Consequently, it has been necessary to adapt the materials used in theturbine to this increase in temperature, by improving techniques forcooling turbine blades (hollow blades), and/or by improving theproperties of such materials in terms of their ability to withstand hightemperatures. This second technique, in combination with the use ofsuperalloys based on nickel and/or cobalt, has led to various solutionsincluding depositing a coating of thermally insulating material known asa thermal barrier.

Under steady operating conditions and with a part that is cooled, theceramic coating makes it possible to establish a temperature gradientthrough the coating with a total amplitude that can exceed 200° C. for acoating that is about 150 micrometers (μm) thick. The operatingtemperature of the underlying metal forming the substrate for thecoating is thus reduced by that gradient, thereby leading to significantsavings in the volume of cooling air that is needed, and to significantimprovements in the lifetime of the part and in the specific fuelconsumption of the turbine engine.

Naturally, in order to improve the properties of the thermal barrier,and in particular its bonding with the substrate, it is possible toinclude an underlayer between the substrate and the coating. Inparticular, it is known to make an underlayer constituted by one or morealuminides, comprising in particular a nickel aluminide optionallyincluding a metal selected from platinum, chromium, palladium,ruthenium, iridium, osmium, rhodium, or a mixture of these metals,and/or a reactive element selected from zirconium (Zr), hafnium (Hf),and yttrium (Y), and/or an alloy of the MCrAlY type, where M is a metalselected from nickel, cobalt, iron, or a mixture of those metals.

Usually, ceramic coatings are deposited on the part to be coated eitherby a spraying technique (in particular plasma spraying), or by aphysical vapor deposition technique, i.e. by evaporation, in particularby electron beam physical vapor deposition (EB-PVD) forming a coatingthat is deposited in an enclosure for vacuum evaporation under electronbombardment).

With a sprayed coating, a zirconia-based oxide is deposited by plasmaspraying type techniques, leading to the formation of a coatingconstituted by a stack of droplets that are molten and then quenched byshock, being flattened and stacked so as to form a deposit that isimperfectly densified and that has a thickness that generally lies inthe range 50 μm to 1 millimeter (mm).

A physically deposited coating, and in particular a coating deposited byevaporation under electron bombardment, leads to a coating that is madeup of an assembly of columns directed substantially perpendicularly tothe surface to be coated, over a thickness lying in the range 20 μm to600 μm. Advantageously, the space between the columns allows the coatingto compensate effectively for the thermomechanical stresses that aredue, at operating temperatures, to differential expansion relative tothe superalloy substrate, and to centrifugal mechanical stresses due torotation of the blades. Parts can thus be obtained having long lifetimeswhen subjected to thermal fatigue at high temperature.

Conventionally, such thermal barriers thus lead to a discontinuity inthermal conductivity between the outer coating of the mechanical part,comprising said thermal barrier, and the substrate of the coatingforming the material that constitutes the part.

Usually, it is found that thermal barriers which lead to a largediscontinuity in thermal conductivity suffer from a high risk ofseparation between the coating and the substrate, and more precisely atthe interface between the underlayer and the ceramic thermal barrier.

At present, it is desired to obtain thermal barrier compositions whichenable mechanical parts to withstand surface temperatures of about 1500°C., i.e. up to about 1300° C. within the substrate. The thermal barrierspresently in use enable mechanical parts to withstand surfacetemperatures of about 1200° C. to 1300° C., i.e. about 1000° C. to 1100°C. within the substrate.

It is known to make use of a thermal barrier that is obtained from abase material constituted by zirconia possessing a coefficient ofexpansion that is close to that of the superalloy constituting thesubstrate, and of thermal conductivity that is quite low.

The present invention relates to the type of coating that is obtained byevaporating a target under an electron beam. The targets used aresubjected to thermal shock when they are irradiated by the electronbeam, which thermal shock can lead to the target breaking, in particularif the target presents defects and/or irregularities. When the targetbreaks, it is no longer usable in practice, since it is no longercapable of delivering material by evaporation in regular manner.

Patent application EP 1 055 743 relates to a material that can bedeposited by electron beam evaporation, in which it is desired tocompensate, at least in part, for the change in volume of the materialdue to the thermal expansion that occurs when the temperature rises dueto the irradiation, by means of the 4% volume reduction that is inducedby the phase transition between monoclinic zirconia which transformsinto tetragonal zirconia as temperature rises from 500° C. to 1200° C.More precisely, action is taken on a broad distribution of particlesizes for the powder of monoclinic structure so as to ensure that thiscompensation takes place over a quite broad range of temperature values.

EP 1 055 743 also provides for the presence of monoclinic zirconia at aconcentration of 25% to 90%, or preferably in the range 40% to 85%, forthe purpose of improving ability to withstand thermal shock. As in DE 4302 167, this improved resistance to thermal shock comes from theappearance of microcracks during the phase transition between thetetragonal phase and the monoclinic phase while temperature is falling,which microcracks are capable of absorbing the thermal shock energy soas to prevent cracks from propagating, and thus prevent the materialfrom breaking. According to EP 1 055 743, the two above-mentioned rolesof the monoclinic zirconia serve to increase resistance to thermalshock.

According to EP 1 055 743, the targets are unusable outside those rangesof values. More precisely, when the monoclinic phase content of thezirconia is less than 25%, thermal expansion during evaporation iscompensated to a lesser extent by the volume reduction during phasetransformation, and the proportion of microcracks is too small, therebylimiting resistance to thermal shock. When the monoclinic phase contentof the zirconia is greater than 90%, the volume expansion induced by thephase change between tetragonal zirconia transforming into monocliniczirconia during the cooling that follows the temperature rise inherentin evaporation is too great, thereby leading to cracks (seams orquenching cracks) greatly reducing the strength of the target andpossibly leading to breakage thereof.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a solution that makesit possible to obtain a composite target in the form of a barconstituted by one or more mixtures of ceramic powders includingzirconia and at least one zirconia stabilizer, and designed to beevaporated under an electron beam, which can be implemented easily inreproducible manner, giving rise to a target of good quality.

An object of the present invention is thus to enable a ceramic target tobe obtained for evaporating under an electron beam in order to obtain adeposited ceramic layer having the same composition as that of thetarget.

To this end, the present invention provides a composite target in theform of a bar made of ceramic powders for the purpose of beingevaporated under an electron beam, the target comprising zirconia and atleast one zirconia stabilizer, wherein said zirconia stabilizer isincluded at a molar content lying in the range 2% to 30%, and whereinsaid zirconia is formed by more than 90% of a monoclinic phase.

Contrary to the teaching of EP 1 055 743, it has been found,surprisingly, that a content of greater than 90% of monoclinic zirconiais entirely compatible with the looked-for target properties of strengthwhen cold and resistance to thermal shock.

The invention makes it possible to obtain targets having mechanicalproperties that are optimal for this application, i.e. fairly weak so asto provide good resistance to thermal shock, while nevertheless beingstrong enough to allow the target to be handled without being damaged.

It has been found that the evaporation behavior of targets presentingthe monoclinic phase of zirconia at a content of greater than 90% isless sensitive to variations in other characteristics of the targets,and in particular pore diameter, specific gravity, and porosity.

For example, it has been observed that variations in pore diameter overthe range 0.4 μm to 1.5 μm at constant specific gravity, or variationsin specific gravity of the range 2.8 to 3.3 at constant pore diameter,lead to results that are identical whether in terms of behavior duringdeposition by evaporation or in terms of the characteristics of thecoatings that are obtained by such deposition.

The above-mentioned variants in the characteristics of targets can arisewhen changing powder batch, and thus changing mean diameter and specificsurface area of the particles in the powder. From one batch of powder toanother, small variations in particle size or in specific area arefrequently observed.

Thus, by choosing to provide a monoclinic phase of zirconia at a contentof greater than 90% in the target, it is possible to use the same methodof fabricating targets without modifying fabrication parameters, and inspite of the varying characteristics of the powders used.

Preferably, in the target, said zirconia is formed by more than 98% of amonoclinic phase.

In a preferred disposition, said stabilizer comprises at least oneelement belonging to the group formed by oxides of rare earth, tantalumoxide, and niobium oxide. In this respect, the term rare earth is usedto mean the lanthanides (lanthanum, cerium, praseodymium, neodium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, and lutetium), and scandium andyttrium.

Preferably, the specific gravity of the target is less than 3.9, andpreferably lies in the range 2.5 to 3.3.

Also preferably, the target presents a mean pore diameter d₅₀ of lessthan 2 μm (preferably lying in the range 0.2 μm to 1.5 μm, and moreparticularly in the range 0.4 μm to 1.2 μm) with porosity of 30% to 50%.

These two parameters constituted by mean pore diameter and by targetspecific gravity have an influence on the mechanical strength of thetargets, and on their behavior during evaporation, and in particulartheir resistance to thermal shock. These factors can be controlled byappropriately selecting the initial raw materials and by selecting thefabrication parameters.

The present invention also relates to a target presenting a compositionthat varies along its height.

The present invention also provides a method of fabricating a compositetarget in the form of a bar as defined above, made of ceramic powdersand for the purpose of being evaporated under an electron beam.According to the invention, the method comprises the following steps:

a) preparing at least a first mixture having a first composition from abinder and a powder comprising zirconia and at least one zirconiastabilizer, said zirconia being formed by more than 90% of a monoclinicphase;

b) introducing said mixture into a mold;

c) compacting the mixture in said mold; and

d) baking the compacted mixture at a temperature of less than 1500° C.

The stabilization temperature is a function of the powder system underconsideration: it is generally below 1500° C., and preferably lies inthe range 900° C. to 1100° C.

The baking temperature should be low enough to conserve a monoclinicphase content of greater than 90% in the target, i.e. to limit thestabilization of the zirconia which is accompanied by bridges beingestablished between the particles of powder, leading to a drop inresistance to thermal shock.

The use of pure zirconia powder, i.e. powder that is not stabilized,during step a) enables targets to be fabricated having a wide variety ofcompositions and at lower cost. In the trade, zirconias are commonlysold that have been stabilized with the more conventional stabilizingagents constituted by Y₂O₃, MgO, CaO, and CeO₂ at fixed contents (inparticular at 3%, 4%, or 5% molar content for Y₂O₃ relative to thequantity of ZrO₂). It is expensive to synthesize powders stabilized withother types of stabilizing agent (e.g. rare earth oxides) or withspecial contents: synthesizing is performed either by chemical means(expensive precursor), or by physical means (calcining a mixture, thengrinding and screening in order to obtain the desired grain size range).In addition, using one or more mixtures of raw powders enables thechemical composition to be controlled in all sections along the lengthof the target, thus making it possible to vary the content of a givencompound within the thickness of the coating, depending on requirements.The method thus avoids synthesizing a multitude of stabilized zirconiamixtures in advance when making composite targets made up of a pluralityof segments having different compositions.

The binder is preferably aqueous, i.e. including water, but it may alsoinclude an organic binder such as polyvinyl alcohol.

In a preferred disposition, said stabilizer comprises at least oneelement belonging to the group made up of rare earth oxides, tantalumoxide, and niobium oxide. In this respect, the term rare earth is usedto mean the lanthanides (lanthanum, cerium, praseodymium, neodium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, and lutetium), and scandium andyttrium.

Preferably, said step a) comprises preparing at least one second mixturehaving a second composition made up of a binder and powders includingzirconia and at least one zirconia stabilizer, said zirconia beingconstituted by more than 90% of a monoclinic phase, and step b) includesintroducing the first mixture and the second mixture in succession, soas to obtain a target of composition that varies along its height.

Preferably, said ceramic powders present a mean particle diameter lyingin the range 5 μm to 30 μm, or a specific surface area of less than 10square meters per gram (m²/g), and preferably a specific surface arealying in the range 3 m²/g to 8 m²/g.

In another disposition, said ceramic powders present a mean particlediameter of less than 5 μm and step a) includes a substep of calciningthe powder prior to incorporating it in the binder.

Such a calcining step makes it possible to readjust the grain size ofthe powder to a value lying in the range 5 μm to 30 μm.

The present invention also provides a ceramic thermal barrier having lowthermal conductivity and high thermomechanical strength, formed byevaporating a target of the above-specified type under an electron beam,deposited on a superalloy substrate.

The present invention also provides a ceramic coating comprising abonding underlayer, a first ceramic layer based on yttrified zirconiacontaining a molar content of yttrium oxide lying in the range 4% to12%, and a second ceramic layer formed by a thermal barrier as definedin the preceding paragraph, said first ceramic layer being situatedbetween said bonding underlayer and said second ceramic layer.

Finally, the present invention also provides a mechanical part made ofsuperalloy, including a ceramic coating having a thermal barrierobtained from a target of the above-specified type.

In particular, the following advantageous dispositions can beimplemented in accordance with the invention with respect to themechanical part:

-   -   it further includes a bonding underlayer on which said ceramic        coating is deposited;    -   said bonding underlayer is constituted by an alloy suitable for        forming a protective alumina layer by oxidation;    -   said bonding underlayer is constituted by an alloy of the MCrAlY        type, where M is a metal selected from nickel, cobalt, iron, and        mixtures of these metals;    -   said bonding underlayer is constituted by a nickel aluminide        optionally containing a metal selected from platinum, chromium,        palladium, ruthenium, iridium, osmium, rhodium, or a mixture of        these metals, and/or a reactive element selected from zirconium        (Zr), hafnium (Hf), and yttrium (Y); and/or

said ceramic coating further includes, on said underlayer, a ceramiclayer based on yttrified zirconia having a molar content of yttriumoxide lying in the range 4% to 12%.

Other advantages and characteristics of the present invention willappear on reading the following description of embodiments of targetsthat are given in non-limiting manner.

EXAMPLE 1

The target was prepared under the following conditions:

-   -   mixing ZrO₂ (100% monoclinic, mean particle diameter d₅₀=25 μm,        and specific surface area of 1.20 m²/g) and Y₂O₃ powder (4%        molar relative to the quantity of ZrO₂,mean particle diameter        d₅₀=5.16 μm), these powders having purity >99.9%;    -   adding a binder in the form of polyvinyl alcohol at a content of        3.5% by weight relative to the mixture as a whole;    -   introducing the mixture into a mold;    -   applying a pressure of 100 bars (isostatic pressing); and    -   baking at 1300° C. for 1 hour.

The target obtained in that way presented specific gravity of 3.27, amean pore diameter d₅₀=2.04 μm, porosity of 44%, a monoclinic crystalphase content of 91.7%, a thermal expansion coefficient of 6.8×10⁻², anda total volume shrinkage of 3.7%.

Nevertheless, it was not possible to make any deposit since thetarget-forming bar cracked on preheating up to 850° C.

EXAMPLE 2

The target was prepared under the following conditions:

-   -   mixing ZrO₂ (100% monoclinic, mean particle diameter d₅₀=16.7        μm, and specific surface area of 4.4 m²/g) and Y₂O₃ powder (4%        molar relative to the quantity of ZrO₂, mean particle diameter        d₅₀=0.99 μm), these powders having purity >99.9%;    -   adding a binder in the form of polyvinyl alcohol at a content of        3.0% by weight relative to the mixture as a whole;    -   introducing the mixture into a mold;    -   applying a pressure of 1600 bars (isostatic pressing); and    -   baking at 1000° C. for 1 hour.

The target obtained in that way presented specific gravity of 3.11, amean pore diameter d₅₀=0.75 μm, porosity of 44%, a monoclinic crystalphase content of 100%, a thermal expansion coefficient of 0.78×10⁻², anda total volume shrinkage of 7.4%.

A deposit was made successfully with that bar, creating a ceramiccoating forming a thermal barrier.

EXAMPLE 3

The target was prepared under the following conditions:

-   -   mixing ZrO₂ (100% monoclinic, mean particle diameter d₅₀=21.8        μm, and specific surface area of 7.7 m²/g) and Dy₂O₃ powder (12%        molar relative to the quantity of ZrO₂, mean particle diameter        d₅₀=2.97 μm), these powders having purity >99.9%;    -   adding a binder in the form of polyvinyl alcohol at a content of        4.0% by weight relative to the mixture as a whole;    -   introducing the mixture into a mold;    -   applying a pressure of 1600 bars (isostatic pressing); and    -   baking at 1000° C. for 1 hour.

The target obtained in that way presented specific gravity of 3.14, amean pore diameter d₅₀=0.40 μm, porosity of 49%, a monoclinic crystalphase content of 95%, a thermal expansion coefficient of 0.55×10⁻², anda total volume shrinkage of 9.5%.

A deposit was made successfully with that bar, creating a ceramiccoating forming a thermal barrier.

1. A composite target in the form of a bar made of ceramic powders forthe purpose of being evaporated under an electron beam, the targetcomprising zirconia and at least one zirconia stabilizer, said zirconiastabilizer being included at a molar content lying in the range 2% to30%, and wherein, after sintering of said target, said zirconia in thetarget contains more than 90% of a monoclinic phase.
 2. A targetaccording to claim 1, wherein the stabilizer comprises at least oneelement belonging to the group formed by rare earth oxides, tantalumoxide, and niobium oxide.
 3. A target according to claim 1, presentingspecific gravity of less than 3.9.
 4. A target according to claim 1,presenting mean pore diameter d50 less than 2 μm.
 5. A target accordingto claim 1, presenting porosity of 30% to 50%.
 6. A target according toclaim 1, presenting composition in zirconia and stabilizer that variesalong the height of the target.
 7. A target according to claim 1wherein, after the sintering of said target, said zirconia in the targetcontains at least 91.7% of a monoclinic phase.
 8. A target according toclaim 1 wherein, after the sintering of said target, said zirconia inthe target contains at least 95% of a monoclinic phase.
 9. A targetaccording to claim 1 wherein, after the sintering of said target, saidzirconia in the target contains more than 98% of a monoclinic phase. 10.A target according to claim 1 wherein, after the sintering of saidtarget, said zirconia in the target contains 100% of a monoclinic phase.